244 PHYSICS: C. BARUS Proc. N. A. S. 
moved normally towards the lower, a distance of y, the equations reduce to 
- AV = 4^Qd ,, y/A(K"d' + K'd") = const, jn, 
Ay being the potential difference thus produced and measured at the U- 
tube electrometer taken as small in capacity in comparison with the 
electrophorus. 
Q = Q' + Q". Hence, as a first approach, the y, n locus is a parabola. 
For instance in the following example the insulation loss amounted to not 
more than 2 fringes in 10 minutes at full charge. The pitch of the microm- 
eter screw being .1 cm., the upper plate was conveniently discharged 
when d' = 1 cm. above the hard rubber surface. Large fringes (about 
1.5 scale parts) were installed. The fringe displacements (n) observed on 
lowering and raising the plate are shown in figure 2. The outgoing and 
incoming series practically coincide. 
4. Specific Inductive Capacity. — In equation (7) if the space d' is filled with 
air, K' = 1. On the other hand if a plate of some insulator like glass is 
inserted of thickness d' g 
d' = d' g + d' a 
where d' a is the thickness of the air layer. Moreover if K g is the specific 
inductive capacity of the insulator 
d'/K' = d' a + d' g IK g 
If, therefore, in the absence of the insulator, y is the downward displace- 
ment of the upper plate which gives the same fringe displacement n, and 
hence the same V as the insertion of the insulator plate, the resulting 
equations eventually reduce to 
K e =d'J{d e -y) 
To determine the specific inductive capacity of a given insulating plate, the 
electrophorus is discharged at a convenient distance, d f , between plate 
and hard rubber face. The insulator (K g ) is then inserted (noting the 
fringe displacement n) and withdrawn. The fringes must return to zero, 
showing that no charge has been imparted by the friction of the insulator. 
The upper plate is now depressed (y) on the micrometer screw until the 
same fringe displacement n is obtained. The operation is quite rapid; 
nevertheless the results so obtained were usually too large. Dielectric 
hysteresis was looked for, but could not have exceeded a fringe breadth. 
5. Absolute Values. — The comparison of the U-tube with three different 
Elster and Geitel Electroscopes, the latter all standardized in volts, is given 
in figure 3 and is as linear within the reading error. The U-tube results 
were computed by equation A, measuring d from the mercury surface M' 
in figure 1 to the electrode C' } with allowance for the K of glass plate. They 
are about four times too large. When, however, the measurement of d 
was made from the top of the glass plate to the electrodes, the results of the 
two instruments practically coincided. Hence the thin glass plate here 
acts like a conductor. The charge is transferred to its top face. 
